Function of Transcriptional mediator

果蝇Fushi Tarazu基因被FTZ-F1转录激活需要两个额外的因子MBF1和MBF2。我们从家蚕的后部丝腺中提纯了这些介体，并克隆了这些因子的编码基因。MBF1似乎是一种由146个氨基酸残基组成的碱性蛋白，在从酵母到人类的各种物种中都是保守的。它与FTZ-F1相互作用，增加FTZ-F1与DNA的结合，这种相互作用需要FTZ-F1盒的C末端一半在FTZ-F1的DNA结合域内。MBF1还与TATA元件结合蛋白TBP直接接触，MBF2是一个由92个氨基酸残基组成的碱性蛋白。天然MBF2通过N-糖苷键产生多糖。MBF2与MBF1形成异源二聚体，并与TFIIA的β亚基结合，单独加入到转录混合物中时，可激活多种启动子的转录。相反，在MBF1和FTZ-F1存在的情况下，它选择性地激活FTZ启动子的转录素。非选择性激活和选择性激活都需要TFIIA。根据这些结果，我们提出了FTZ-F1转录激活的以下模型。MBF1是连接FTZ-F1和TBP之间的桥梁因子，并将阳性辅因子MBF2招募到携带FTZ-F1结合位点的启动子上。MBF2通过与TFIIA的β亚基相互作用，以FTZ-F1结合位点依赖的方式激活转录。

Transcriptional activation of the Drosophila fushi tarazu gene by FTZ-F1 requires two additional factors MBF1 and MBF2. We have purified these mediators from posterior silk glands of the silkworm and cloned cDNA's coding for the factors. MBF1 appears to be a basic protein consisting of 146-amino acid residues and conserved across species from yeast to human. It interacts with FTZ-F1 and increases the FTZ-F1 binding to DNA.This interaction requires the C-terminal half of the FTZ-F1 box within the DNA-binding domain of FTZ-F1. MBF1 also makes a direct contact with TATA element-binding protein TBP.MBF2 is a basic protein consisting of 92-amino acid residues. Native MBF2 bears polysacharides through N-glycoside bonds. MBF2 forms a heterodimer with MBF1 and also binds to beta-subunit of TFIIA.When MBF2 alone was added to transcription mixtures, it activated transcription from various promoters. In contrast, it selectively activated transcriptoin from ftz promoter in the presence of MBF1 and FTZ-F1. Both non-selective and selective activation requires TFIIA.From these results, we have proposed the following model for transcriptional activation by FTZ-F1. MBF1 is a bridging factor that connets between FTZ-F1 and TBP and recruits the positive cofactor MBF2 to a promoter carrying the FTZ-F1-binding site. MBF2 activates transcription in a FTZ-F1-binding site dependent manner through its interaction with the beta-subunit of TFIIA.